1. Field of the Invention
The present invention relates to a measurement control method of an injection molding machine that performs a measurement by screw rotation and ends the measurement when the screw retracts to a preset measurement ending position.
2. Description of the Relevant Art
In general, an injection cycle of an injection molding machine has a measurement process and an injection process; and during the measurement process, measurement is performed by rotating a screw while measurement control is performed in which measurement is ended when a screw is retracted to a preset measurement ending position. However, although various types of measurements are taken during the measurement process such as rate control, pressure control, and position control, increasing the control precision for a series of controls in the measurement process is extremely important for securing consistent injection molding quality and obtaining an injection molded object of high quality; and various measurement control methods have conventionally been disclosed.
For example, in Japanese Patent Gazette (Publication) No. 6 (1994)-61800, a control method for retracting a screw to a preset number of screw rotations while rotating the screw, and for stopping the screw at a preset measurement ending position, and a measurement control method (measurement control device) of an injection molding machine for receiving a screw retraction rate detected by a screw rate detection means and a screw position detected by a screw position detection means, and for calculating the number of screw rotations so as to stop the screw at a measurement ending position by a predetermined calculation formula, and for sending out the calculated number of screw rotations as a rotation drive command, is disclosed. Further, in Japanese Laid Open Patent Publication No. 2004-154988, a measurement control method for an injection molding machine (measurement method) for controlling the screw rotation rate as a screw rotation rate command by deriving the screw rotation rate proportional to the position deviation between the set measurement ending position and the current screw retraction position then by correcting the screw rotation rate based on the pressure deviation between the set resin pressure and the currently detected resin pressure for the screw rotation rate after a screw is retracted to a set screw position in the vicinity of the set measurement ending position, is disclosed.
However, the conventional measurement control method for an injection molding machine as described above has the following problems.
First, as described in aforementioned Publication No. 6 (1994)-61800, when adopting a control method that detects the screw position and the screw retraction rate and calculates the number of screw rotations to stop the screw by a measurement ending position while making the calculated number of screw rotations to be a rotation drive command, a significant amount of time is required until a screw arrives at the measurement ending position as the rotation rate of a screw near to the measurement ending position approaches zero without limit. Since shortening the cycle time to something that is beneficial to raising the control precision of the screw position cannot be devised, a limitation occurs in the extent to which injection molding efficiency and mass productivity can be raised while being extremely inconvenient relative to realizing high speed injection molding.
Further, as shown in aforementioned Publication No. 2004-154988, when adopting a measurement control method which makes a screw rotation rate command by deriving a screw rotation rate that is proportional to the position deviation between the set measurement ending position and the current screw retraction position while correcting it by the pressure deviation between the set resin pressure and the currently detected resin pressure after a screw is retracted to a set screw position in the vicinity of the set measurement ending position, control in the vicinity of the set measurement ending position is only position control, thereby creating a fixed subject of control. Therefore, it is difficult to achieve securing responsiveness and stability in something that is beneficial to raising control precision of the screw position and allows back pressure control in a minimal length area while back control requires adjustments in the screw rotation rate as well as inviting complications to control.
Further, although measurement control is required to secure a high degree of measurement precision in reliably stopping both the screw rotation and retraction at a measurement ending position (measurement ending position), in neither case is consideration given to reliably stopping both the screw rotation and retraction, and nor do they especially provide sufficient measures for the formation of, for instance, a thin optical disk recently requiring a high degree of measurement precision.